1. Field of the Invention
The present invention relates to apparatus and method for mixing/diluting, generating, and/or transferring of process chemicals. More particularly, the present invention provides improved process and apparatus for the precise mixing/diluting all forms of chemicals and, especially, ultra-high purity chemicals for use in a variety of industries, such as in the manufacture of semiconductor wafers and similar products.
2. Description of the Prior Art
The inventions of the parent applications are directed to process and apparatus for the transfer and delivery of high-purity chemicals from a bulk source to one or more end-use stations. As is explained therein, in many applications in industry today it is extremely important to maintain process chemicals free of virtually all contaminants. For instance, in the semiconductor industry the purity of chemicals, such as sulfuric acid, hydrogen peroxide, and ammonium hydroxide, used in semiconductor wafer production must be pure on level of approximately 25 (or fewer) particles per milliliter with a particle size of less than a fraction of a micron. As a result of these purity standards, many conventional methods of chemical transfer and delivery, such as paddled pumps and similar devices, have proven completely unsatisfactory.
Of further concern in these industries is that many of the chemicals employed are toxic, chemically aggressive, and/or require special conduit material, and must be carefully handled. In order to assure adequate purity and worker safety, it is extremely important that such chemicals be transferred, stored, and dispensed in a closed system, with minimal contact with the environment or workers.
Prior to the inventions of the parent applications, generally one of two methods were employed to effectuate high-purity chemical transfer. The first method is a "pumped delivery." In this method a positive displacement pump, usually an air powered double diaphragm type, is employed to provide both lift at a suction inlet from a bulk source of the chemicals and simultaneous pressure at the output to the end-user. The chemical is lifted from a chemical drum, driven through a pump, and pushed out to the point of use.
Although the pumped delivery system is widely employed, it is far from satisfactory. This system is capable of producing only minimal lift from the chemical bulk source--usually on the order of only a few pounds per square inch. Moreover, the system is replete with contamination problems: the rapidly expanding and Contracting of the pump diaphragm material (e.g. TEFLON.RTM.) causes mechanical degradation, with the degradation by-products (many of which being too small to filter with state-of-the-art filtration equipment) entering the chemical process stream; further, the rapid action of the pump (usually greater than 60 cycles per minute) creates massive impulses in the system with a resulting pulsed flow which forces particles through filters--thus rendering the filters far less effective. Finally, the mechanical shock and vibration inherent in this system creates constant maintenance problems, such as leaks.
The other system which is generally used addressed only some of these problems. In a "pump/pressure delivery" system, a positive displacement pump is again employed to provide lift from the bulk source of chemicals. However, the chemicals are delivered to an intermediate vessel from which inert gas pressure is used to motivate chemical to the use areas.
Although the pump/pressure system is better controlled and is more conducive to use of filters to assure chemical purity, it still has serious drawbacks in a sub-micron chemical environment. Again, lift provided by a double diaphragm pump is restricted. Further, such pumps are prone to degradation--with the by-products entering the chemical stream. Finally, the use of a single pressure vessel for delivery results in non-continuous delivery--constraining the volume of each delivery to "batch" sizes based on the size of the pressure vessel. If demand exceeds the volume of the pressure vessel, further delivery must be "queued" while the pump refills the pressure vessel. Alternatively, pressure from the pump that is equal to or greater than the pressure of the delivery vessel must be applied to the delivery vessel to supplement or refill it during demand; this further compounds filtration and maintenance problems.
The inventions disclosed in the parent applications solve all these problems. In those inventions, a combination of vacuum and pressure is used to transfer chemical smoothly from a bulk source, through one or more intermediate pressure/vacuum vessels ("PVV"), and to one or more end-use stations. First, a vacuum pump is used to establish a vacuum in one of the PVVs to draw chemicals into the PVV. Once a PVV is filled, the vessel is then pressurized to motivate chemical to an end-use station, to another PVV, or for recirculation back to the bulk source. The elimination of pumps from all chemical conduits in the system avoids the problems of degradation and contamination.
As is explained in the parent applications, the advantages of this improved transfer and delivery apparatus include: even (i.e. non-pulsed) flow through the system, reducing maintenance problems and allowing far more efficient use of filters; built-in redundancy to assure constant chemical supply and fail-safe operation; and electronic controls to monitor and maintain all aspects of system operation automatically.
In light of these substantial advantages of a vacuum/pressure transfer and delivery system, applicants believe that similar advantages can be achieved in a vacuum/pressure system for taking chemicals from multiple bulk sources and automatically mixing them (e.g. combining two or more process chemicals or diluting one or more process chemicals with water or other chemical) prior to delivery to the end-user.
Although liquid mixing systems are known, none addresses the contamination concerns of a high-purity environment. Most existing systems employ conventional fluid transfer means (e.g. pumps or water line pressure) to fill two or more metered vessels. From these vessels, the liquids to be mixed are then transferred, usually by pumps, to a mixing vessel and then to a storage facility or user. Examples of such existing systems are illustrated in U.S. Pat. Nos. 4,960,295, issued Jun. 1, 1976, to Horak, 4,019,528, issued Apr. 26, 1977, to Tyrrell, 4,215,719, issued Aug. 5, 1980, to Laar et al., and 4,823,987, issued Apr. 25, 1989, to Switall. None of these teaches means to assure that high purity chemicals will not be contaminated on a sub-micron level by the mixing apparatus itself.
Additionally, none of the existing mixing systems provides a simple yet effective method of accurately mixing chemicals in precise volumes. Although it is common to employ multiple metered vessels to measure the amount of each chemical to be mixed, with sensors typically used to cease the flow of liquid to the metered vessel once it is filled, none of these systems provides means to amplify the sensors'accuracy in order to assure very precise measurement of the volume of each vessel. The use of other metering methods, such as highly accurate flow meters or similar devices, may address some of these concerns, but are generally undesirable due to their expense, fragile nature, and/or possible contamination risks. As a result, none of the existing diluting/mixing systems is considered fully satisfactory in providing accurate mixing of high purity chemicals required by many industries.
Accordingly, it is a primary object of the present invention to provide improved apparatus and method for accurate mixing of chemicals from two or more bulk sources.
It is a further object of the present invention to provide an apparatus and method that includes, or readily interfaces with, means to transfer and deliver chemical from the bulk sources to end-use stations.
It is another object of the present invention to provide an apparatus and method that includes means to transfer and mix high-purity process chemicals from bulk sources and deliver them reliably and without contamination to end-use stations.
It is yet another object of the present invention to provide an apparatus and method that includes a simple and relatively inexpensive means to yield extremely precise volumetric measurement of chemicals.
It is an additional object of the present invention to provide an apparatus and method that employs a vacuum-pressure transfer and delivery system so as to generate even flow and negligible mechanical shock in the system.
These and other objects of the present invention will become evident from review of the following specification.